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This invention relates to an electronically commutated brushless D.C. motor and the pulse width modulated (PWM) control signal generating circuitry used to provide control signals for this motor.
Brushless D.C. motors include a rotating permanent magnet rotor, a stator carrying field coils, and a drive circuit for sequentially exciting the field coils with digital pulses, thereby creating electronic commutation. Electronically commutated motors eliminate or reduce the disadvantages inherent in motors with mechanical structures for a commutator. Specifically, RFI (radio frequency interference) losses and EMI (electro-magnetic induction) losses are reduced or eliminated. Brush and armature maintenance is eliminated, and power consumption attributed to armature-brush arcing is also eliminated.
Typically, an electronic timing drive circuitry incorporating active electronic components, i.e., transistors, FETs (field effect transistors), MOSFETs (metal oxide semiconductor field effect transistors) has been used to provide PWM drive pulses. The PWM drive signal circuit is connected directly to a power supply. Drive pulse generation has been synchronized with rotor position by the incorporation of monitoring or feedback circuitry, including the use of optical position sensors and/or magnet position sensors, such as Hall effect devices.
Many such DC motors are used in electronic applications that may utilize batteries as the energy source. Examples of applications of such battery-powered fans are laptop computers, and telecommunication equipment. More and more, desktop computing devices and non-portable telecommunication equipment are being backed-up by arrays of batteries. Therefore, this equipment must be designed to operate on battery power.
When batteries are used to power such equipment, it is desirable that this equipment be very efficient, so that there is a minimum of energy consumption. This has become an important design requirement for cooling fans that are used in such battery-powered equipment. It has become important that these fans, and the motors that power them, be designed for efficiency and for minimum energy consumption. It is also desirable that fan motors used for cooling telecommunications equipment provide maximized airflow to pressure characteristics, with a minimum of noise and DC power consumption.
In systems situations, such as telecommunications systems, batteries are used in the system and charged while the system and the battery driven equipment is in operation. Sometimes, the batteries are charged to a higher voltage than the nominal value required for the equipment. Examples of this are the lead-acid batteries used in telephone centers. In the presence of power shortages, or blackouts, these batteries may discharge to a voltage value below the rated voltage for the system. As an example, battery banks rated at a nominal working voltage of 48 volts, may be permitted to swing between 40-56 volts. Under severe load conditions, additional batteries may be added to a bank (which increases its operating voltage). Under such conditions, a nominal bank output voltage may be permitted to swing between 60-72 volts.
If a DC fan motor rated at 48 volts were installed in the system where the battery bank operated between 60-72 volts, the motor would consume excessive power. It would also produce and excessive amount of noise. Motor controller electronics and coil driver power transistor switching circuits may be subjected to overheating and burnout. Even when the battery bank operates about its nominal voltage value, there may be a considerable voltage swing to which a motor is subjected. It is therefore desirable to provide a DC fan motor that will maintain a constant speed and operate under minimum power consumption under power supply voltage variations.
While the variations recited above were directed to battery powered systems, voltage variations can occur even with regulated power supplies operating from ac current. Moreover, power surges are not unheard of with these ac powered-dc supplies. Such power surges manifest themselves as voltage variations. Therefore, a constant speed motor circuit is also desirable in ac powered dc supply systems.
PWM control pulses are of fixed height and frequency and are of variable pulse width. As the pulse width varies so does the power delivered to the motor coils. As the power varies, so does motor speed. Erratic motor speeds increase power consumption, can create more heat, and create electrical noise.
Excessive supply voltages and variations in supply voltage, whether originating with batteries or not, will result in changes in the PWM signals produced. These changes have occurred in prior PWM controller circuits when erratic voltage levels, drifting voltage levels, and voltage spikes have been incurred. It is desirable that the effects of these changes be avoided.
In prior circuits, the PWM motor drive is used to switch on and off the incoming energy source in order to control the current (or voltage) supplied to a load (motor coil). Generally higher frequency pulses have in the past been utilized in the generation of PWM control pulses. The frequency used to carry the pulses of variable width (PWM) has been higher than the frequency of the waveform that needed to be controlled.
The PWM technique has been widely used to control the power supply voltage applied to a DC fan motor in order to control fan speed. Chinomi, et al. (U.S. Pat. No. 6,256,181 B1) disclose a pulse width modulated (PWM) motor drive circuit. Chinomi changes the drive pulse rate by controlling (changing) drive pulse width. Erdman, et al. (U.S. Pat. No. 6,271,638 B1) use a capacitor-coupled bridge circuit power supply to further reduce power consumption. Erdman further uses a Hall sensor control of the pulse generator to limit current usage. Erdman uses a stall sensor to deactivate the field coil driver pulse circuitry in the presence of a stall and fault condition.
Horiuchi, et al. (U.S. Pat. No. 5,969,445), have used a brushless motor, incorporating a sensor magnet for sensing rotor position. This sensor magnet provides a feedback signal indicative of each rotor pole position and thereby the rotational speed of the brushless motor. The Horiuchi drive circuit, which creates electronic commutation, incorporates FETs, and obtains power from a power supply containing an AC-to-DC converter. The Horiuchi sensed rotor position signal is used to optimize the drive pulse effects as a function of pulse generation timing (i.e. leading and/or falling edges) verses rotor magnet position.
Schmider, et al. (U.S. patent application Pub. 2001/000 4194), use bi-stable multivibrator circuits for implementing the electronic commutation of the motor""s field magnets. One or more comparator circuits control the switching state of the multivibrator. The Schmider comparator circuit is controlled by a voltage induced in a field coil which has just been deactivated. The need for a separate rotor position sensor is eliminated.
Hall effect devices (Hall generators) have been substitute for the Horiuchi-type magnet sensor (U.S. Pat. No. 6,211,635 B1, Kambe, et al.). Kambe uses a single Hall generator in his motor to determine rotor position and to generate synchronization signals for the drive pulse circuit operation.
In the use of PWM digital drive pulses, a current spike or over voltage can occur at the time of or shortly after each coil current transition time. Shunt or snubber circuits have been used to protect the drive circuitry from these spikes. These devices limit over voltage or over current transients, during or after the operation of switching current (leading edge or falling edge of a pulse) in a field coil. Snubber circuits are shown in Markaran, et al. (U.S. patent application Pub. 2001/0000293 A1). Both Markaran""s field coil switch and his snubber circuit switch are implemented by MOSFETs. When Markaran""s field coil is turned off, by the opening of his in-line switch (drive pulse driver circuit switch), the residual energy in the field coil begins to drain through the snubber diode into his snubber capacitor. When the voltage on that snubber capacitor reaches a certain level, the snubber MOSFET is turned on (the snubber switch is closed) and the voltage on the snubber capacitor begins to drain through the snubber inductor to the positive voltage node (to the power supply).
Alvaro, et al. has taken a different approach to protecting the drive pulse switching circuit (U.S. Pat. No. 6,239,565 B1). He places a capacitor (RC series circuit) between the driver circuit (drive pulse driver circuit switch) and ground (the negative node). This limits the voltage that can appear at the positive node due to back EMF or field coil discharge.
While the use of PWM control signals to control fan operation has been a popular technique, there have been some disadvantages to using this technique. These disadvantages have included that some fan motor controllers require a PWM input, where the user must provide an external signal of variable pulse width. This has required the user to provide an additional external circuit to achieve the desired speed control which adds to the cost and the size of the structure.
Additionally, the high frequency switching which is required for PWM control contributes to switching losses especially at higher DC voltages. Furthermore, the higher frequencies contribute to greater electromagnetic interference. It is desirable to reduce or eliminate these factors from a PWM controller circuit.
A further disadvantage has been that sophisticated circuitry has been required to synchronize the generating (carrier) pulses with the control signal generated by a position sensor, such as a Hall effect sensor (Hall generator). This synchronization is particularly critical at lower motor speeds so that smooth fan operation (motor rotation) occurs. Lastly, where PWM signals are generated within the fan motor circuitry itself, there is incurred increased expense. The generation of accurate and stable PWM signals has generally been an involved task. Previous PWM signal generation circuitry located within the motor circuitry itself has required an external resistance or the input of an externally generated voltage in order to vary not only the pulse width, but also its frequency. In some instances there have been space and heat dissipation constraints.
An object of the present invention is to provide an improved PWM controller circuit for constant speed control of an electronically commutated fan motor, whereby power losses are reduced and energy consumption minimized.
A second object is to provide a constant speed PWM controller circuit that maintains constant motor speed in the presence of power supply voltage variations.
A further object is to provide this PWM controller circuit with constant output in the presence of voltage spikes, erratic voltage levels, and drifting voltage levels.
Another object is to provide this PWM controller circuit output which does not need the input of an additional external circuit of variable pulse width, and does not need high frequency switching to product the PWM control, thereby reducing switching losses and electromagnetic interference.
The objects are realized in a controller circuit for an electronically commutated (brushless) DC motor, having a reduced component count. This controller circuit includes circuitry that generates pulse width modulation (PWM) control signals for the activation of the motor field coil(s). These PWM control signals do not change with variations in supply voltage, nor in the presence of voltage spikes or erratic voltage levels, thereby producing constant speed motor operation. Power losses are reduced and energy consumption minimized. The PWM control signals are generated without the need for high speed switching or external circuitry generated variable pulse with input signals.
The rotation of the rotor""s magnetic field is detected by a Hall sensor (Hall generator circuit) which senses rotor position and speed and provides synchronizing pulses. A pair of Hall effect integrated circuit (IC) implemented output transistors provide the excitation current to a motor coil, bilaterally (in alternating directions) by alternately conducting and non-conducting, with only one output transistor conducting at a time. Depending upon the power capacity selected for these output transistors, which in turn is related to the power needed to drive the motor and the power limitations of the PWM controller circuit, a separate bridge circuit may be utilized.
Where a bridge circuit is used to directly power a motor coil from a power supply, the output transistors are used to trigger the operation of the bridge circuit. That bridge circuit includes power-switching transistors, whose alternating conduction states bilaterally excite a motor coil. The conduction states of the bridge circuit, switching transistors are then controlled by the outputs from the IC output transistors. With small motors, such as those used to drive cooling fans in small electronic devices, such as laptop and desktop computers, the output transistors are connected to directly excite a motor coil. The synchronizing pulses from the Hall sensor are input to the output transistors to synchronize their operation with the instantaneous physical relationship of the rotor poles with a motor coil.
A pulse current sensor circuit senses the change in current direction, i.e., the bilateral operation of a coil. The output of this coil current pulse sensor is input to a monostable multivibrator circuit. The output of this monostable multivibrator circuit is PWM signals, which are sent to the IC output transistors to control their respective conduction states. The pulse current sensor circuit provides a feedback signal to the circuit.
The width of the pulses produced by the monostable multivibrator circuit is established by a resistance circuit, which implements the time constant circuit portion of the monostable multivibrator circuit. This resistance circuit is physically located external to the monostable multivibrator circuitry. It has a manual adjustment whereby a user manually sets the pulse width and therefore the desired motor speed. It also has an automatic adjustment portion, which compensates for variations in supply voltage levels.
The automatic variations in resistance values in this resistance circuit act to compensate for voltage irregularities by providing a consistent time constant within the monostable multivibrator. The pulse width of the PWM signals from the monostable multivibrator is thereby unaffected by changes in supply voltage as there is a compensation for voltage differences.
Supply voltage changes normally, ultimately, change motor speed. The purpose in compensating for supply voltage changes is to keep the motor speed constant. Therefore, implementation of the automatic compensation within the resistance circuit utilizes a feedback signal of instantaneous motor speed. The resistance circuit includes a voltage difference circuit with an output to a voltage controlled resistor. The voltage controlled resistor forms a portion of the time constant circuitry within the PWM monostable multivibrator circuit. The voltage difference circuit measures the difference between a set reference voltage and the output voltage from a peak value detector. This peak value detector is driven from a signal generated from the coil current pulse sensor, which is first processed through a reset monostable multivibrator and then an integrator circuit which inputs the peak value detector.